I thought TDK did a lot to establish that he really WAS in this for the long-haul, and that he really WAS driven by the same things the Batman of the comics was.
But it turns out.... not so much.
Did you even watch The Dark Knight
? Bruce's whole mission in the thing is to put Harvey up on that pedestal so that he (Bruce) can put down the mantle of Batman and be with Rachel. The discussion at the restaurant (where Harvey makes his oft-quoted "You either die a hero or live long enough to see yourself become the villain"), and Bruce's later speech to Rachel at the fundraiser about giving up being Batman all support this.
And yes, he chose at the end to take the blame for Harvey's death, but I don't know that I agree that his doing that necessarily equates to his desire or intention to continue being Batman forever.